Ink jet recording is an image recording technique wherein small ink droplets are expelled and made to adhere to a recording paper so as to form dots. In ink jet recording there is little noise, full color images are easily obtained, and as no developing or fixing is required, recordings can be made at high speed. In recent years, the technique has been attracting considerable attention due to its ability to make perfect copies of color images displayed on a cathode ray tube or other device, or of diagrams and color originals (i.e. its ability to produce hard copies), and it is therefore rapidly coming into general use. In line with the diversification of needs, however, there has been a notable increase in the demand for larger image sizes, higher speeds and higher definition of the recorded image.
From the viewpoint of the aforesaid needs, an ink jet recording paper (referred to hereafter simply as recording paper) is required to have the following properties.
(1) Ink adhering to the recording paper is absorbed without running, and absorbed ink does not smudge.
(2) Ink dots formed on the paper have high contrast, and a bright color tone.
(3) Ink dots are suitably distributed in the direction of the paper surface, the dots being almost round with sharp edges.
(4) The paper has excellent smoothness and gloss so that clear, bright recorded images are obtained.
(5) Recorded images are water-resistant (i.e. if the recorded image on the paper surface gets wet, the ink dots forming the image do not smudge and their color does not run).
To resolve the aforesaid problem (1), non-colloidal silica powders having high ink absorption were incorporated in the coating on the paper (e.g. Japanese Tokkai Sho 55-51583). In this case however, although the ink absorption of the recording paper was improved, its smoothness, gloss and water resistance were inadequate.
To improve the smoothness and gloss of the recording paper, a super calender or gloss calender was applied to the paper surface (e.g. Tokkai Sho 57-167879); alternatively, a thermoplastic resin or particles of same were incorporated in the paper, heat and pressure being applied to the paper after printing so as to dissolve the resin; or again, the paper surface was treated with a plasticizer capable of swelling and/or an organic solvent (e.g. Tokkai Sho 53-50744, Tokkai Sho 59-196285, Tokkai Sho 59-201891, Tokkai Sho 59-204591, Tokkai Sho 59-204592 and Tokkai Sho 59-222381).
These methods succeeded in improving the smoothness and gloss of the recording paper, but its ink absorption properties deteriorated. In particular, in the aforesaid method of incorporating a thermoplastic resin in the recording paper, the treatment of the paper after recording was tedious so that high speed was difficult to achieve, and in addition the manufacturing cost of recording equipment was increased.
To increase the water resistance of recorded images, some proposals mention the incorporation of a basic oligomer in the paper (e.g. Tokkai Sho 60-11389).
This technique does improve water resistance, but a considerable amount of water adheres to the paper when the basic oligomer is coated. The paper therefore becomes creased and wrinkled, and the appearance of the recorded image deteriorates.
To improve ink absorption, smoothness, gloss and water resistance properties of the recording paper, methods of manufacturing ink jet recording paper involving preparation of a cast coated paper have been proposed (e.g. Tokkai Sho 61-209189, Tokkai Sho 62-95285, Tokkai Sho 63-211394, Tokkai Sho 63-264391, Tokkai Sho 63-265680 and Tokkai Hei 1-95214), and good results have been obtained. However, even in those cases, the density, sharpness and roundness of each dot were still not good enough to obtain high quality, high contrast, full color recorded images.